The present invention relates to connecting two tubes through a ball-cup connection. This connection should be tight with regard to diffusion of gasses from the surrounding atmosphere.
In processes at elevated temperatures in furnaces, process tubes are used from ceramic material. Such tubes are connected to feed/discharge tubes outside the furnace or other tubes such as tubes in which a thermocouple or other sensor is introduced. Connection between the two tubes should prevent the ingress of gasses from the surrounding atmosphere into the process tube and leakage of process gasses to the surroundings.
In the prior art ball-cup connections are used to achieve this purpose. When the cup and the ball are dimensioned correctly and are perfectly mating, a high quality seal is achieved. However, in practice it is difficult to control the dimensional tolerances of the cup and the ball. Further, when the ball is larger than the cup, breakage of the cup can easily occur. Therefore the nominal diameter of the ball should always be slightly smaller than the diameter of the cup. Consequently, the ball and cup connection is not a perfect seal. Furthermore, if the ball and the cup are made of different materials some tolerances need to be allowed to accommodate differences in thermal expansion when the furnace is heated from room temperature to process temperature. Because the requirements regarding the leak integrity of such a connection have become more and more stringent, in the art the ball-cup connection is provided with an additional O-ring seal. However, in situations where the temperature of the connection is fairly high, the lifetime of the O-ring is impractical short due to aging of the material of the O-ring,
In U.S. Pat. No. 5,653,479 a ball-cup connection is proposed to connected low pressure chemical vapor deposition (LPCVD) tube to a vacuum system via a vacuum exhaust tube. The cup, attached to the vacuum exhaust tube, contains annular shaped grooves, cut into the inside surface of the cup. Each groove contain a connection to a second vacuum system. When the ball, attached to the LPCVD process tube, is inserted into the cup, the open side of the grooves is bounded by the surface of the ball. The grooves are evacuated by the second vacuum system and this differentially pumped vacuum seal is sufficient to contain a rough vacuum, down to 1 mTorr.
This design has several limitations and disadvantages. The design of the cup is rather complex. All embodiments show three grooves, machined into the cup, which cup is of stainless steel. Stainless steel is not allowed at die inlet side of an oxidation furnace to connect e.g. an outside torch system to the process tube because it would lead to metallic contamination. Cutting the grooves in a ceramic cup, like quartz, is much more complicated and costly than cutting the grooves in stainless steel. Furthermore, for reasons given above, the diameter of the ball will always be smaller than the diameter of the cup. The more so as in the embodiment shown in U.S. Pat. No. 5,653,479 the cup is of stainless steel and the ball of ceramic material. Even when at room temperature the cup and ball fit exactly together, when heated the cup will be larger than the ball because of the larger thermal expansion coefficient of stainless steel as compared to quartz. The actual contact between cup and ball will be a line rather than a plane. Over the line contact, gas transport through diffusion can easily take place. Another disadvantage of the proposed connection is that it is directed towards vacuum applications. When the process tube is at atmospheric pressure, just as the surrounding atmosphere, it is not convenient to apply a vacuum to the connection because then air from outside and process gas from the inside are sucked into the coupling and meet each other there, resulting in potentially undesired reactions. Finally, the design requires a second vacuum system with all its connections and supplies, adding to the complexity of the installation.
It is the object of the invention to avoid the above disadvantages and provide in a high temperature resistant ball-cup connection that prevents the ingress of gasses from the surrounding atmosphere into the process tube and that is in particular suited for atmospheric process tubes.
According to one aspect of the invention a joint assembly is provided for connecting two tubes, one of said tubes comprising at its extremity a cup shaped member and the other of said tubes comprising at its extremity a ball shaped member to be received in said cup shaped member, said cup shaped member being provided with a gas feed channel discharging at one end into an annular groove at the cup/ball interface and at the other end connected to a conduit outside said cup shaped member, said conduit being connected to an inert gas source having a pressure above atmospheric pressure.
Because of the use of overpressure of the inert gas introduced between the parts of the ball joint, inert gas will be flowing from the annular groove through the gap between the cup shaped member and the ball shaped member, in one direction towards the surrounding atmosphere and in the opposing direction towards the interior of the process tube. The supply of inert gas should be such that in the gap an inert gas flow speed is maintained at a sufficiently high level such that back diffusion of atmospheric gasses such as oxygen and water vapor through the ball joint is reduced to an acceptable minimum.
The joint assembly according to the invention can be used in any application wherein two tubes have to be connected to each other and where the connection should be heat resistant and prevent the ingress of gasses from the surrounding atmosphere. More particular this joint is used in a furnace for processing semiconductor wafers.
According to a preferred aspect of the invention the process tube is provided with a ball at its extremity whilst the tube connecting thereto is provided with a cup shaped member. The tube connecting to the furnace tube can be a gas feed/discharge tube. However, the invention can also comprise the connection with a sensor tube. Typically, the process tube comprises a ceramic material like quartz or SiC. Preferably, the connecting tube comprises also a ceramic material like quartz or SiC, although for some applications it is possible that the connecting tube comprises a metal like stainless steel.
According to a further aspect, the invention relates to a method for operating of a furnace, comprising a ceramic material furnace tube in which a substrate is processed and through which a processing gas is flowed, a ceramic material feed/discharge tube connected to said furnace tube through a ball joint and through which said processing gas and possible reactants are fed/discharged, wherein between the cup and the ball of said ball joint an inert gas having a super atmospheric pressure is introduced.